1. Field of the Invention
This invention relates generally to acoustic beamforming. More particularly this invention relates to using angular spectral dispersion in a passive or active acoustic system to form spatially discriminating beams.
2. Description of the Prior Art
The dispersion of a frequency spectrum into angular space is commonly seen in optics. An optical prism spreads white (multi-frequency) light into a spectrum that is angularly dispersed in frequency. Optical dispersion can also be accomplished with a grating. A blazed grating is commonly used to create optical dispersion of reflected light. A blazed grating is characterized by the blaze angle, which is the angle between the effective surfaces of the profile and the general plane of the grating.
Radar systems also employ this general principal in a technique called frequency-scanned arrays. See Skolnik, Radar Handbook, McGraw-Hill, Ch. 13 (1970). This technique employs delay lines on the antenna staves to provide the proper phase shifts so that the frequency determines the steering angle of the main beam of the array. These radar systems differ from phased array radar because phased array radar allows full use of the frequency band for purposes beyond beam scanning.
In the prior art, active and passive acoustic systems such as sonar and ultrasound have employed the well-known techniques such as phased or tine-delayed arrays to steer beams. Phased arrays are limited by their complexity, Each stave must be sampled individually, and be accompanied by the requisite hardware. This results in acoustic systems having large size, weight and power requirements. Consequently, the sonar must suffer either limited performance or limited applicability when the footprint of an acoustic imaging system must be small as in sonar systems in small underwater vehicles.
RE U.S. Pat. No. 034,566 to Ledley, discloses an ultrasound technique that creates multiple acoustic beams with different frequencies by utilizing a plurality of independent, narrow-band transducers. Each transducer is arranged and driven such that its beam covers one beam of the sonar's field of view. Because each transducer (or line of transducers) works independently, this technique does not form a coordinated array of sensors across the angular dimension utilizing the multiple frequencies. Therefore, it does not provide the improved azimuth discrimination gains typically associated with an array of transducer elements, but instead is limited to the resolution provided by a single transducer element. In the mechanically fixed embodiment this technique requires a separate transducer for every beam. Therefore, the available aperture is severely under-utilized and the number of beams limited. In the rotating embodiment this technique is limited by the mechanical scanning speed and the added mechanical complexities of the rotating projector.